Majorana fermions and half-integer thermal quantum Hall effect in a quantum magnet

by Prof. Yuji Matsuda (Kyoto University)

Wednesday 25 May 2022, 10:00 → 11:00 Asia/Shanghai

ONLINE

Abstract

When topology is combined with strong electron correlations, exotic phenomena can arise.  One of the most prominent example is the fractional quantum Hall effect, in which the constituent particles are electrons but the quasiparticles carry fractions of the electron charge.  Recently, Kitaev quantum spin liquid (QSL) has aroused a great interest, because exotic quasiparticles, such as Majorana fermions and non-Abelian anyons, emerge as a result of fractionalization of electron spins.  Kitaev model represents a spin-1/2 on a honeycomb lattice interacting through bond-dependent Ising ferromagnetic couplings.

Recently, spin-orbit Mott insulator a-RuCl₃ is drawing much attention as a promising candidate material hosting a Kitaev QSL state. In a-RuCl₃, evidence for the spin fractionalization has been reported by several measurements, including specific heat, Raman scattering, and inelastic neutron scattering.  Moreover, the half-integer quantized thermal Hall conductance provides evidence for the formation of a topologically nontrivial state consistent with the Kitaev model [1]. The half-integer thermal Hall plateau appears even for a magnetic field with no out-of-plane components (planar thermal Hall effect). The measured field-angular variation of the quantized thermal Hall conductance has the same sign structure as the topological Chern number of the Majorana band in the Kitaev model [2]. The specific heat measurements for different in-plane rotations of an applied magnetic field revealed strongly angle-dependent low-energy excitations. While the energy gap, which increases with the magnetic field as D~H³, opens for H||a,  gapless Dirac cones are retained for H||b. These results also provide strong support for the band dispersion of itinerant Majorana fermions in magnetic fields [3]. 

These results provide direct signatures of topologically protected chiral currents of charge neutral Majorana fermions at the edge and no-Abelian anyons in the bulk of the crystal.  

 

[1] Y. Kasahara et al., Nature 559, 227 (2018).

[2] T. Yokoi et al. Science 373, 568 (2021). 
   [3] O. Tanaka et al. Nature Physics 18, 429 (2022).

Biography

Yuji Matsuda received his Ph.D. in Physics from the University of Tokyo (Japan) in 1987 and became a research associate at Department of Pure and Applied Science, the University of Tokyo. He became an associate professor in 1993 at Hokkaido University (Japan) after spending two years at Princeton University (USA) as a postdoctoral fellow. He moved to Institute for Solid State Physics, University of Tokyo, as an associate professor in 1997, and became a full professor at Kyoto University in 2004. He is a condensed matter experimentalist with interests in electronic and magnetic properties of solids.His current research interests include strongly correlated electron systems, in particular exotic superconductivityand quantum spin systems.

Division

Condensed Matter

Other information

Vedio link: https://vshare.sjtu.edu.cn/open/6963689d6b67743f5e6d9ed327f27e0f